1. Field of the Invention
The present invention relates to an improvement in photoelectric conversion devices for solar cells and for power sources in various electric appliances. More particularly, the present invention relates to a photoelectric conversion device provided with an improved back reflection layer comprising a specific zinc oxide material which enhances the utilization efficiency of incident light in the active semiconductor region and provides improved photoelectric conversion efficiency.
2. Related Background Art
There are known a number of photoelectric conversion elements for solar cells and for power sources in various electric appliances which have been put into practical use. Among those photoelectric conversion elements, there is a solar cell made of a single crystal silicon material, commonly called the "single crystal silicon solar cell." This single crystal silicon solar cell is highly reliable and high in photoelectric conversion efficiency. The single crystal solar cell is produced by ion implantation or thermal diffusion of an impurity into a single crystal substrate of silicon (Si) or gallium arsenide (GaAs), or by epitaxial growth of an impurity-doped layer on said single crystal substrate. However, the production cost is unavoidably costly because of using such a specific single crystal substrate and the cell is extremely difficult to make in a large area.
In any case, in order to make solar cells usable as practical power sources, it is essential that a large area solar cell be industrially mass-produced at a reduced cost. This requirement is not attained by the single crystal solar cell.
In recent years, attention has been focused on the so-called thin film solar cells comprising amorphous silicon (hereinafter referred to as "a-Si") semiconductors or compound semiconductors such as CdS/CuInSe.sub.2 and the like. Although they are inferior to the single crystal silicon solar cell, they have various advantages over the single crystal silicon solar cell. For example, their semiconductor layer can be formed on a relatively inexpensive substrate such as glass or stainless, their manufacturing process is relatively simple, they can be designed to be of a large area, and they can be provided at a reduced cost.
Various studies have been made of thin film solar cells to improve their photoelectric conversion efficiency. Particularly, in order to heighten the performance of such thin film solar cells in terms of the utilization efficiency of incident light, it has been proposed to include in the thin film solar cell a metal reflection layer (or a back reflection layer) at the substrate surface capable of reflecting light such as sunlight not absorbed by the thin film semiconductor layer and return it to the thin film semiconductor layer, thereby improving the photoelectric conversion efficiency.
The thin film solar cell according to this proposal can include a configuration in which light is impinged from the substrate side and another configuration in which light is impinged from the side opposite the substrate. The former configuration comprises a transparent substrate, a thin film semiconductor as an optically active semiconductor layer disposed on said transparent substrate, and an electrode layer composed of a metal having a high reflectance such as Ag, Al, or Cu disposed on said thin film semiconductor. The latter configuration comprises a substrate composed of a metal or alloy having a high reflectance and a thin film semiconductor as an optically active semiconductor layer disposed on said substrate.
However, both of these configurations are still problematic in that reflected light is not sufficiently utilized because the thin film semiconductor is relatively thin, hence it is difficult to sufficiently improve the solar cell electrical characteristics.
In order to improve this situation, there is a proposal to interpose a transparent conductive layer comprising a material having a given optical property, such as zinc oxide (ZnO), between the thin film semiconductor (the optically active semiconductor layer) comprising an a-Si material and the metal layer comprising a metal such as Ag, Cu, or Al. The aim is to improve the light utilization efficiency by virtue of multiple-interference effects of the transparent conductive layer while preventing light externally impinging the optically active semiconductor layer from being reflected at the metal layer having a high reflectance and reflected outside the optically active semiconductor layer.
FIG. 3(A) is a graph showing measured results of the reflectance of each of the metals Ag, Cu and Al as the metal layer as a function of the wavelength of the incident light in an a-Si thin film solar cell without a transparent conductive layer. FIG. 3(B) is a graph showing measured results of the reflectance of each of the metals Ag, Cu and Al as the metal layer as a function of the wavelength of the incident light in an a-Si thin film solar cell with such a transparent conductive layer.
Japanese Patent Publication No. 60-41878/1985 (hereinafter referred to as Literature 1) discloses an a-Si thin film solar cell having a transparent conductive layer interposed between a silicon semiconductor photoelectroc conversion layer and a back electrode composed of a metal having a high reflectance to light of a wavelength from 0.3 to 2 .mu.m. The transparent conductive layer prevents formation of an alloy layer at the interface between the silicon semiconductor layer and the back electrode.
U.S. Pat. No. 4,532,372, (hereinafter referred to as Literature 2) and U.S. Pat. No. 4,598,306, (hereinafter referred to as Literature 3) disclose a thin film photovoltaic device of configuration similar to the thin film solar cell described in Literature 1. The transparent conductive layer is composed of a material having a great electrical resistivity to prevent excess current flow between the opposite electrodes even in the case where a short-circuited portion occurs in the active semiconductor region, thereby improving the reliability of the photovoltaic device.
However, each of the transparent conductive layers described in Literatures 1 to 3 causes multiple-interference effects of the incident light in the direction parallel to the direction of the incident light by virtue of the difference between the refractive index of the active semiconductor layer or region and that of the transparent conductive layer at the interface between them. This improves the utilization efficiency of reflected light to a certain extent but is not satisfactory.
U.S. Pat. No. 4,419,533, (hereinafter referred to as Literature 4) discloses a photovoltaic device having an incident radiation directing means comprising a metal layer having a surface with a textured structure and a transparent conductive layer having an uneven surface provided with irregularities disposed on the textured surface of the metal layer, in which the transparent conductive layer serves as a back reflection layer. In Literature 4 incident light is scattered at the surface through which light is impinged or at the interface between the back reflection layer and the semiconductor active region. The incident light is directed and confined in the semiconductor active region by virtue of light trapping effects, wherein the light is effectively utilized.
Shown in FIG. 1 is a typical example of a thin film solar cell having such a back reflection layer. In FIG. 1, reference numeral 101 indicates an electroconductive substrate. Reference numeral 102 indicates a metal layer having a high reflectance which is laminated on the electroconductive substrate 101. The metal layer 102 has an uneven surface. When the electroconductive substrate 101 is composed of a material having a sufficiently high reflectance and has a randomly roughened surface, the metal layer 102 can be omitted. Reference numeral 103 indicates a layer capable of serving as a transparent conductive layer (back reflection layer) which is laminated on the uneven surface of the metal layer 102. The transparent layer 103 has an uneven surface which conforms to the uneven surface of the metal layer 102. Reference numeral 104 indicates a thin film semiconductor layer composed of an amorphous material such as an a-Si material and having a three-layered structure 105, 106, 107 with a pin junction which is laminated on the uneven surface of the transparent layer 103. Reference numeral 108 indicates a transparent electrode which is disposed on the thin-film semiconductor layer 104. Reference numeral 109 indicates a collecting electrode in a desired shape such as a comb-like shape.
In this thin film solar cell, the transparent layer 103 is composed of a material which is transparent to the light passing through the thin film semiconductor layer and has a desired electric resistivity. The three-layered structure of the thin film semiconductor region 104 comprises an n-type semiconductor layer 105 composed of an amorphous material having photoconductivity, an i-type semiconductor layer 106 composed of an intrinsic amorphous material having photoconductivity, and a p-type semiconductor layer 107 composed of an amorphous material having photoconductivity, laminated in this order from the substrate side. When the thin film semiconductor layer 104 is relatively thin, the surface thereof is usually uneven, as shown in FIG. 1.
In the configuration shown in FIG. 1 in which the back reflection layer is disposed, a significant improvement in the photoelectric conversion efficiency is expected. However, in practice, the photoelectric conversion efficiency in this configuration is not as good as originally expected and is still not satisfactory.
Thus, there is an increased demand for a further improved thin film solar cell in which the utilization efficiency of incident light and the photoelectric conversion efficiency are further improved.